The present invention relates to a vacuum-operated power brake booster comprising a booster piston that is sealed in relation to the vacuum booster housing by a rolling diaphragm. The booster comprises a piston rod coupled to a brake pedal for the actuation of a double valve. By means of the valve a working chamber of the power brake booster is alternatively connectible to vacuum or to higher differential pressure. A first valve is formed by a valve seat at the booster piston and by a poppet valve preloaded in the direction of the valve seat, while a second valve is formed by the same poppet valve and by a valve piston connected with the piston rod.
A vacuum booster as shown in German printed and published patent application No. 29 42 416, comprises a booster piston which subdivides the interior of a booster housing into a first working chamber, that is inwardly connected to a vacuum source, and into a second working chamber. A valve cylinder projects from the back side of the booster piston and contains at its front side a first annular valve seat, with a valve piston being slidably arranged in the valve cylinder. The valve piston carries a second annular valve seat which is encompassed by the first valve seat and is coupled to a driving rod. The valve cylinder also contains a valve member which is urged by a preload member against the first and the second valve seat. The first and the second working chamber will establish communication through a gap between the first valve seat and the valve member upon retraction of the valve piston. Upon advance movement of the valve piston, the second working chamber will be connected with the atmosphere via a clearance between the second valve seat and the valve member. Between the booster housing and the driving rod in this vacuum booster is a stop which confines the travel of retraction of the driving rod such that the clearance between the first valve seat and the valve member in the inactive position of the driving rod is smaller than this clearance is when the driving rod is to be retracted.
This known vacuum-operated power brake booster bears the disadvantage that, although the lost travel can be reduced until response of the device, it cannot be eliminated, since the tolerances of the individual components constituting the control influence the lost travel with a view to preventing leakage of the device. Likewise in other comparable known power brake boosters, special arrangements make the valve piston approach the control housing sealing seat by about the "response lost travel".